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A state of an item, where the item is not able to perform as required.

The duration of the fault may range from negligible to permanent. There are two main types of faults.

 Type 1 fault is a fault that occurs as a consequence of a failure. The failure causes a transition from a functioning state into a fault, which is also called a failed state. In Example 3.4, the failure of the light bulb left the bulb in a state where it cannot give light. In this example, the bulb has to be replaced to function again.

 Type 2 fault is a fault that is introduced in the item due to human error or misjudgment in the specification, design, manufacture, transportation, installation, operation, or maintenance of the item. This type of fault enters the item without any preceding item failure and is a dormant fault that remains hidden until the item is activated or inspected. A type 2 fault is also called a systematic fault. A software bug is a typical example of such a fault. Another example is faults caused by design errors or installation errors.